2007-09-12

Bureau of Meteorology starts thinking La Niña

With the exception of the Southern Oscillation Index (SOI), all ENSO indicators have reached a point that, should they persist at these levels until the end of the year, 2007 will be referred to as a La Niña.

Most critically, the near-equatorial Pacific Ocean has continued to cool both on and below the surface, the Trade Winds remain stronger than normal across the western to central Pacific, and cloudiness in the equatorial Pacific is reduced. Together, these indicators suggest the atmosphere and ocean may now be reinforcing each other; a critical component in sustaining La Niña conditions for any period of time.

However, if a La Niña does evolve during spring, it would be late by historical standards. In the past, most significant La Niña events were established by winter's end, with widespread above-average rain falling over Australia's eastern half. With a late-developing La Niña, this typical rainfall response is not as likely as in past episodes.

Moreover, Australia's climate may continue, at least in the short term, to be influenced by the unusual state of the oceans to the north, and particularly northwest, of the continent. These have been cooling strongly since June when, historically, they would have been expected to warm as other indicators became more La Niña-like. This pattern inhibits the formation of northwest cloudbands, which are a major source of winter and spring rain for central and southeastern Australia during La Niña years.